Ecm?

I am having trouble with my 98 model 88. I started missing and check engine light flashing. Checked the code and got P300, multiple cylinder misfire. It had been about 50K since last tuneup, so gave it a new set of plugs, wires, and 3 new coil packs. Test drove it and everything seemed fine. I have about 80 mile drive to work and the next morning about 40 miles into trip it started missing again. It is missing on at least 2 cylinders. If you let it cool off for 6 or 8 hours or overnight it will run fine until it heats up good under the hood. The ecm is under the hood on this model and I had a 92 Regal do this same thing to me. I was told that it had a crack in the board and when it got hot the crack opened up and caused the misfire. Would you try a used ecm?

ECM not ICM? you could try sparying Cool IT from radio shack on the ECM when it starts missing. (a can of 134 will do the same thing) IF it stops missing that is it. And yes I would buy a used ECM, They are blamed far more often then they go bad.... please read on

Best guess is a thermal problem (I have seen this several times over the years) the darn thing will run fine and then, when it gets warm, it starts missing. It is probably the ICM (but it could be the crank or cam sensor.) This problem shows up all the time even when using junkyard parts. They check good but don’t exactly test them for 40+ miles. I have seen the ICM do things such as... start like crap but run fine..... Start fine idle fine, then miss at speed or at idle after a 5 10 minet warm up.......

Replaced ICM and it missed right from startup. Any help?

Tough problem. Losing a cylinder or two after warmup could be fuel injectors, engine controller, cam or crank sensor. Obviously something electronic is getting hot and breaking down. Pinpointing it is probably going to be a pain in the azz. Just too many things can cause a misfire. Good luck

Sounds like the typical early warning sings of a failing crank sensor, especially with the P0300 DTC.